1. Field of the Invention
The present invention relates to a plasma processing apparatus and a processing method and, more particularly, to a plasma processing apparatus and a processing method such as: a plasma CVD apparatus and a film forming method for a crystalline or non-monocrystalline functional deposition film effective for an electrophotographic photosensitive device serving as a semiconductor device, an image input line sensor, an image-pickup device, or a photomotive device; a sputtering apparatus and a film forming method which can preferably form a semiconductor device, an insulating film serving as an optical element, or a metal wire; or an etching apparatus and method for a semiconductor device.
2. Related Background Art
As a plasma process used in forming a semiconductor or the like, various methods are used depending on applications. For example, an apparatus and a method using the characteristics of a plasma, such as film formation for an oxide film, a nitride film, and an amorphous-silicon-based semiconductor film using a plasma CVD method; film formation for a metal wiring film or the like using a sputtering method; or a micropatterning technique or the like using etching. In addition, in recent years, a demand for improvement of film quality and processing performance becomes strong, and various devices are examined. In particular, since a plasma process using a high-frequency power has stable discharging and can be applied to an insulating material of an oxide film or a nitride film, the plasma process is widely applied.
Conventionally, the oscillation frequency of a discharging high-frequency wave power source used in a plasma process such as plasma CVD is generally 13.56 MHz. A plasma CVD apparatus generally used in deposition film formation is shown in FIG. 13. FIG. 13 shows a film formation apparatus for a cylindrical amorphous-silicon film (to be referred to as an a-Si film hereinafter) for a cylindrical electrophotographic photosensitive material. A film forming method for an a-Si film will be described below with reference to FIG. 13.
The film formation apparatus shown in FIG. 13 is constituted such that, in a reaction vessel 101 which can be reduced in pressure, a cylindrical cathode electrode 102 electrically insulated from the reaction vessel 101 and a cylindrical film-formed substrate 103 serving as a counter electrode are arranged by an insulating material 113. The film-formed substrate 103 is heated to a predetermined temperature by a heater 105 arranged in the film-formed substrate 103, and is held on a substrate holder 104 having a rotating mechanism driven by a motor 112. A high-frequency wave power source 106 is connected to the cathode electrode 102 through a matching circuit 107. An evacuating means 108 for evacuating the reaction vessel 101 and a gas supply means 109 for supplying a gas into the reaction vessel 101 are attached to the film formation apparatus.
In order to form an a-Si film by the above apparatus, the reaction vessel 101 is evacuated by the evacuating means 108 to a high degree of vacuum, and the gas supply means 109 feeds a source gas such as a silane gas, disilane gas, a methane gas, or an ethane gas or a doping gas such as a diborane gas to keep the pressure in the reaction vessel 101 in the range of several 10 mTorr to several mTorr. The high-frequency wave power source 106 supplies a high-frequency wave power of 13.56 MHz to the cathode electrode 102 to generate a plasma between the cathode electrode 102 and the film-formed substrate 103, thereby decomposing the source gas. At this time, an a-Si film is deposited on the film-formed substrate 103 heated to about 200.degree. C. to 350.degree. C. by the heater 105.
A deposition rate for obtaining an a-Si film which satisfies the performance of an electrophotographic photosensitive material by the film forming method described above is the maximum of about 6 .mu.m/hour. When the deposition rate is higher than 6 .mu.m/hour, the characteristic feature of the photosensitive material cannot be obtained. When the a-Si film is generally used as an electrophotographic photosensitive material, the a-Si film requires a thickness of at least 20 .mu.m to 30 .mu.m to obtain charging performance, and a long period of time is required to manufacture an electrophotographic photosensitive material.
In recent years, the report (Plasma Chemistry and Plasma Processing, Vol 7, No. 3, (1987) p.267-p.273) of a plasma CVD method using a parallel-plate-type plasma CVD apparatus and a high-frequency wave power source of 13.56 MHz or higher has been made. This report shows the probability that a deposition rate can be increased without degrading the performance of a deposition film by making a discharge frequency higher than the conventional frequency of 13.56 MHz. An attempt to increase the discharge frequency has been widely examined in sputtering or the like.
The present inventors used the conventional plasma CVD method and apparatus described above and a high-frequency wave power having a frequency higher than a conventional discharge frequency of 13.56 MHz to increase a deposition rate of a high-quality film.
As a result, when the frequency was increased, as an object, a high-quality film could be manufactured at a deposition rate higher than a conventional deposition rate. However, when the discharge frequency of 13.56 MHz was used, the following problem is newly posed. More specifically, when the discharge frequency was increased, a plasma is eccentrically located to make the deposition rate nonuniform. As a result, in a substrate to be processed such as an electrophotographic photosensitive material having a relatively large area, a film thickness nonuniformity (for example, a film thickness nonuniformity of .+-.20% or more in an electrophotographic photosensitive material) is generated.
Such a film thickness nonuniformity poses a serious problem when not only an electrophotographic photosensitive material but also a crystalline or non-monocrystalline functional deposition film used an image input line sensor, an image-pickup device, a photomotive device, or the like is formed. In another plasma process such as dry etching or sputtering, when a discharge frequency is increased, similarly, nonuniform processing occurs. For this reason, the high-frequency wave power having a frequency higher than 13.56 MHz cannot be practically used without problems.